1. Field of the Invention
This invention relates generally to methods of manufacturing noble metal (Ru, Rh, Pd, Ag, Re, Os, Ir, Pt) layers, such as for use in integrated circuits (IC) and magnetic recording media.
2. Description of the Related Art
Ruthenium metal is considered to be one of the most promising materials for capacitor electrodes of dynamic random access memories (DRAMs) that have for example Ta2O5 and/or (Ba,Sr)TiO3 (BST) dielectrics. Ruthenium is also a potential electrode material for nonvolatile ferroelectric memories. Although platinum has been widely used as an electrode material, many disadvantages are connected to that concept, For instance, it is very difficult to pattern platinum layers by etching and platinum catalyses the dissociation of O2 into atomic oxygen. The formed oxygen thereof diffuses into the underlying barrier, which gets oxidised and forms a resistive layer. On the contrary, ruthenium films can be easily patterned by etching and they prevent oxygen diffusion by forming RuO2, which has good conductivity. Furthermore, because of its large work function, Ru is an interesting electrode material for the future CMOS transistors where SiO2 will be replaced by high-k dielectrics. Though Ru, and for the same reason Ir, are the best candidates in what comes to the oxygen diffusion barrier properties, other noble metals, like Pt and Pd, are still considered as viable candidates for the above applications. With reference to the definition of noble metal, Encyclopedia Britannica states that a noble metal is any of several metallic chemical elements that have outstanding resistance to oxidation, even at high temperatures; the grouping is not strictly defined but usually is considered to include rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold; i.e., the metals of groups VIIb, VIII, and Ib of the second and third transition series of the periodic table of elements.
In addition to electrode applications, thin Ru films find potential use in the fixture in magnetic recording technology where the ever-increasing storage densities set increasing demands on both the read and write heads and recording medium (E. E. Fullerton, Solid State Technology, September 2001, p. 87). In anti-ferromagnetically coupled recording medium, for example, a three atomic layer thick Ru film is used for separating two ferromagnetic layers. In a longer term, perpendicular recording systems (magnetization perpendicular to the film plane) are expected to replace the current in-plane or longitudinal media. To create a high performance recording media with the magnetization perpendicular to the film plane, multilayer structures composed of ultra-thin (typically less than 5 atomic layers thick) magnetic and nonmagnetic layers have been suggested. Here Ru and Pd, for example, could be employed as nonmagnetic materials. An evident challenge for these magnetic recording media applications is how to control the film thicknesses at an atomic layer level uniformly over large substrate areas.
The current metallisation technology of integrated circuits is based on electroplated copper. However, a successful electrodeposition process requires an appropriate seed layer on the diffusion barrier material. Typically copper itself, most often deposited by physical vapour deposition methods, is used as a seed layer material. Chemical methods, like chemical vapour deposition and atomic layer deposition, which could provide better step coverage for the copper seed layer, usually suffer from a poor copper to diffusion barrier adhesion. Further, a general problem related to copper seeds is their easy oxidation, which necessitates a reduction step in the beginning of the electrodeposition process. As noble metals do not oxidise easily on their surface, they can serve as good seed layers for copper electrodeposition.
Currently, Ru films are deposited either by sputtering or by chemical vapour deposition (CVD). ALD processes for depositing Ru films have not been reported, although the characteristics of thin films deposited by ALD, especially excellent step coverage (conformality), accurate and simple thickness control and large-area uniformity, are very valuable features in the above mentioned applications.
The main problem in the development work of depositing metals by ALD has been a lack of effective reducing agents, since the metal precursors applicable in ALD are typically compounds, where the metal is at a higher oxidation state (M. Ritala and M. Leskela, Atomic Layer Deposition, in Handbook of Thin Film Materials, Ed. H. S. Nalwa, Academic Press, San Diego (2001), Vol. 1, Chapter 2, p. 103). A common strategy has been to look for reducing agents that, besides reducing the metal, remove the ligands of the metal compound intact, most typically in a protonated form. The most simple of such a reaction is the process where hydrogen radicals are used as the reducing agent (A. Sherman, U.S. Pat. No. 5,916,365):MLn(g)−>MLn−x(chemisorbed), where x=0 . . . n−1MLn−x(chemisorbed)+(n−x)H(g)−>M(s)+(n−x)HL(g)
Other reducing agents studied for ALD include disilane, diborane, hydrogen, formaldehyde and elemental zinc. In the latter case, zinc removes the halide ligands in the form of volatile zinc halide, e.g. ZnCl2.
Aoyama et al. (Jpn. J. Appl. Phys. 38 (1999) pp. 2194-2199) investigated a CVD process for depositing ruthenium thin films for capacitor electrode purposes. They used bis-(cyclopentadienyl)ruthenium (Ru(Cp2)) as ruthenium precursor and O2 as reactive gas for decomposing Ru(Cp2) gas. The growth temperature was varied from 230 to 315° C. and the growth rate was 25 nm/min at 315° C. However, carbon and hydrogen were incorporated as harmful impurities in the deposited films, thus increasing the resistivity of the film. Furthermore, the general limitations of the CVD method, such as problems related to achieving good large area uniformity and accurate thickness control, still remain. In addition, it is hard to obtain good step coverage and high film purity at the same time.